1. Field of the Invention
The present invention relates to a semiconductor device having a photoelectric conversion element, a manufacturing method and an operation method thereof.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A large number of semiconductor devices generally used for detecting electromagnetic waves are known, and for example, semiconductor devices having sensitivity to ultraviolet rays to infrared rays are referred to as optical sensors. A light sensor which has sensitivity to a visible light region with a wavelength of 400 nm to 700 nm is particularly referred to as a visible light sensor, and a large number of visible light sensors are used for devices which need illuminance adjustment, on/off control, or the like depending on a human living environment.
In particular, in a display device, ambient brightness is detected to adjust display luminance of the display device. This is because unnecessary power consumption can be reduced by detecting ambient brightness and obtaining appropriate display luminance. For example, such an optical sensor for adjusting display luminance is used for a cellular phone or a personal computer.
In addition, as well as ambient brightness, luminance of a back light of a display device, particularly, a liquid crystal display device is also detected by an optical sensor to adjust luminance of a display screen.
In such an optical sensor, a photodiode is used for a sensing part. A chip which has two terminals and is formed by combining an amplifier circuit including a transistor with a photodiode is disclosed in Patent Document 1.
Further, an optical sensor including an amplifier circuit formed of a transistor including an oxide semiconductor layer is disclosed in Patent Document 2.